Body implantable electrical leads form the electrical connection between a pulse generator, such as a cardiac implantable medical device (IMD), and body tissue, such as the endocardium, which is to be electrically stimulated. As is well known, the leads connecting IMDs with the heart may be used for pacing or for sensing electrical signals produced by the heart, or for both pacing and sensing in which case a single lead serves as a bidirectional pulse transmission link between the IMD and the heart. An endocardial type lead, that is, a lead which is inserted into a vein and guided therethrough into a cavity of the heart, typically includes at its distal tip an electrode designed to contact the endocardium, the tissue lining the inside of the heart. The lead further includes a proximal end carrying an electrical connector assembly adapted to be received by a receptacle in the IMD. A flexible cable or coil conductor surrounded by an insulating sheath couples an electrical terminal on the electrical connector assembly with the electrode at the distal tip. For bipolar stimulation and/or sensing, a similar connection may be provided for a ring electrode disposed proximal of the tip electrode.
To prevent displacement or dislodgement of the tip electrode and to maintain the necessary stable electrical contact between the tip electrode and the body tissue, the electrode must be firmly anchored relative to the tissue. For example, an electrode-carrying, distal end portion of a lead body may be configured to bias an electrode into engagement with the target body tissue such as that of a vessel in the coronary sinus region of the heart and thereby passively fix the electrode's position. Another passive fixation mechanism makes use of tines or nubs projecting from the distal end portion of the lead body. Such projections engage the trabeculae within a chamber of the heart or the wall of a vessel receiving the lead.
Another type of lead, sometimes referred to as an active fixation lead, typically includes a pointed, helical element extendable and retractable relative to the lead's tip and thereby adapted to be screwed into the cardiac tissue, typically the endocardium, to be stimulated. In this fashion, the position of the electrode-carrying distal end portion of the lead body is mechanically stabilized by positively anchoring the lead tip so that it remains securely in place during the lifetime of the implant. Current active fixation leads employ helices having lengths in the range of 1.8 to 2.0 mm when full extended from the lead's distal tip.
The fixation helix may itself function as a tip electrode (typically the cathode) in which case it is conventionally coupled by means of an electrical conductor to a rotatable pin terminal on the connector assembly. Rotational torque applied to the connector pin at the proximal end of the lead is transmitted via the conductor to the helix electrode which is thereby screwed into the endocardium. Removal of the screw-in electrode from the heart tissue is effected by counter rotation of the connector pin. Thus, in a lead having a screw-in helix electrode, the coil conductor is used not only as a conductor for electrically coupling the connector pin with the helix electrode, but also as a tool for extending or retracting the helix electrode relative to the distal end of the lead during lead fixation or removal by rotating the connector pin.
Whether the screw-in helix is electrically active or not, the degree of extension of the helix relative to the lead tip must be verifiable by the implanting physician. The current practice is to employ fluoroscopy as a visual feedback mechanism to ascertain whether the helix is fully extended. In aid of such visual confirmation, a high-density, radiopaque, metal collar is typically incorporated in the distal tip of the lead. The collar thus serves as a fluoroscopic helix extension marker. In addition, the electrical conductivity of the collar allows it to be used to sense electrical signals generated by the cardiac tissue adjacent to the distal tip of the lead. The collar may thereby function as a source electrode for mapping localized heart activity prior to the deployment of the electrically active helix electrode. Thus, such mapping helps find, in a non-traumatic manner, a location for installing the helix electrode to optimize sensing and stimulation thresholds. Where the helix is electrically active so as to serve as an electrode, the helix and mapping collar are typically electrically connected in parallel to collectively function as the cathode.
The visual, fluoroscopic confirmation of helix extension has certain limitations. It neither guarantees (1) that the fully extended length of the helix is embedded in the heart tissue, that is, that no part of the helix remains exposed, nor (2) that the distal end of the lead is perpendicular to the local endocardium, that is, that the lead tip is in full engagement with that tissue. Thus, it would be desirable to eliminate having to rely exclusively on visual, fluoroscopic confirmation. Chronically, with fibrotic growth around an exposed portion of the helix, an implanted lead would be susceptible to both mechanical instabilities (in the form of micro- and macro-dislodgements) and electrical instabilities manifested by sensing amplitude decreases, capture threshold increases, and/or electrode impedance decreases.
Another issue associated with active fixation leads is that different physicians prefer different tip electrode impedances (defined generally as the resistance to current flow from the electrode to the heart tissue). While some physicians prefer impedances in the 500 ohm range others seek impedances greater than 1,000 ohms. However, current technologies limit the impedance values that active fixation leads can provide.